The present disclosure generally relates to subterranean stimulation operations and, more specifically, to treatment fluids and methods for acidizing a siliceous material.
Treatment fluids can be used in a variety of subterranean treatment operations. Such treatment operations can include, without limitation, drilling operations, stimulation operations, production operations, remediation operations, sand control treatments, and the like. As used herein, the terms “treat,” “treatment,” “treating,” and grammatical equivalents thereof will refer to any subterranean operation that uses a fluid in conjunction with achieving a desired function and/or for a desired purpose. Use of these terms does not imply any particular action by the treatment fluid or a component thereof, unless otherwise specified herein. More specific examples of illustrative treatment operations can include, for example, drilling operations, fracturing operations, gravel packing operations, acidizing operations, scale dissolution and removal operations, sand control operations, consolidation operations, and the like.
Acidizing operations may be used to stimulate a subterranean formation to increase production of a hydrocarbon resource therefrom. During an acidizing operation, an acid-soluble material in the formation matrix can be dissolved by one or more acids to expand existing flow pathways in the subterranean formation or to create new flow pathways in the subterranean formation. Acid-soluble precipitation damage (i.e., scale) may be removed from the subterranean environment in a related manner. Illustrative substances within the formation matrix that may be dissolved by an acid include, but are not limited to, carbonate materials, siliceous materials, ferrous or ferric materials, or any combination thereof. Introduction of an acidizing fluid to a subterranean formation may take place at matrix flow rates without fracturing of the formation matrix, or at higher injection rates and pressures to fracture the formation matrix (i.e., an acid-fracturing operation).
Carbonate formations can contain minerals, such as calcite or dolomite, which comprise a carbonate anion and a metal counter ion. When acidizing a carbonate formation, the acidity of a treatment fluid alone can often be sufficient to consume the carbonate anion and thereby affect dissolution of the carbonate mineral. Both mineral acids (e.g., hydrochloric acid) and organic acids (e.g., acetic acid and formic acid) can be used to acidize a carbonate formation, often with relatively similar degrees of success. The reaction of such acids with carbonate minerals can generate wormholes and other permeability-enhancing features in the formation matrix. The heterogeneous lithology of carbonate formations can also facilitate the generation of such permeability-enhancing features during an acidizing operation, such as through differential etching and uneven surface dissolution. The increased formation permeability may improve production of a hydrocarbon resource from the formation.
Siliceous formations can include minerals such as, for example, zeolites, clays, and feldspars. As used herein, the term “siliceous” will refer to any substance having the characteristics of silica, including silicates and/or aluminosilicates. The mineral acids and organic acids that are usually effective for dissolving carbonate minerals are generally ineffective for affecting dissolution of siliceous minerals. In contrast, hydrofluoric acid, another mineral acid, can react very rapidly with siliceous materials to promote their dissolution. Additional mineral acids or organic acids may be used in combination with hydrofluoric acid in order to maintain a low pH state as the hydrofluoric acid spends upon reacting with the siliceous material. Unlike the case of carbonate mineral-acidizing operations, the rapid reaction rate of hydrofluoric acid with siliceous minerals can discourage differential etching to form wormhole-like structures and other permeability-enhancing features within the formation matrix. Instead, the hydrofluoric acid usually reacts proximate to its location of first contact with the siliceous mineral, such as in the near-wellbore area. As a result, deeper penetration of the hydrofluoric acid into the formation matrix is typically precluded and the stimulation effect is relatively minimal.
In addition, many siliceous, sedimentary minerals, such as shale, sandstone and mudstone, can have low native permeability values that may further discourage deep penetration of an acidizing fluid into the formation matrix. In these and many other unconventional reservoirs, native permeability values may be below about 0.1 millidarcy, often residing in the nanodarcy range, thereby making these reservoirs highly impermeable to fluid flow. Although fracturing operations may be used to create a fracture network de novo and/or to expand an existing fracture network in a low-permeability siliceous material, ideally stimulating production as a result, further stimulation through an acidizing operation may be difficult to achieve conventionally due to the aforementioned issues associated with the high reactivity of hydrofluoric acid toward siliceous minerals. As used herein, the term “fracture network” will refer to a series of interconnected conduits within a subterranean matrix material that are collectively in fluid communication with a wellbore. The interconnected conduits will also be referred to herein as “fractures,” and they may be naturally occurring, manmade, or any combination thereof. Fractures may be created or extended upon introducing a fluid to a wellbore at or above a fracture gradient pressure of the subterranean matrix material.